The present invention relates to a method for homogeneous catalytic hydrogenation of polymers containing ethylenic unsaturation. More particularly the present invention relates to a novel method for preferentially hydrogenating unsaturated bonds remaining in polymers of conjugated dienes.
Polymers obtained by polymerizing or copolymerizing conjugated dienes are widely utilized for commercial purposes. These polymers have unsaturated double bonds as residues in their polymer chain. While under some circumstances the unsaturated double bonds are advantageous such as for vulcanization purposes, they are a disadvantage as far as stability to weathering conditions and oxidation is concerned. In particular block copolymers obtained from conjugated dienes and monovinylidene aromatic monomers are highly useful items of commerce but find limited application in the preparation of impact resistant resins for use in exposed applications.
In U.S. Pat. Nos. 4,501,857 and 4,673,714 suitable homogeneous polymer hydrogenation catalysts comprising bis-(cyclopentadienyl)titanium compounds are disclosed.
It is additionally previously known in the art to use certain Ziegler-type catalysts in the hydrogenation of polymers. The catalysts are obtained by causing organic acid salts of nickel, cobalt, iron or chromium or acetyl acetone salts to react with a reducing agent such an organic aluminum compound in a solvent. Disadvantageously, it is known that such Ziegler-type catalysts suffer from a defect in selectivity as to the hydrogenation of unsaturated double bonds of conjugated diene units relative to aromatic unsaturation contained in vinyl aromatic units. Under the conditions which permit thorough hydrogenation of the unsaturated double bonds of the conjugated diene units, the aromatic nucleus portion of the copolymer is inevitably hydrogenated to some extent.
A class of bis(pentamethylcyclopentadienyl)lanthanide catalysts has recently been disclosed in U.S. Pat. Nos. 4,668,773, 4,716,257 and 4,801,666. Such organolanthanide catalysts are highly useful in the polymerization of olefins and also in the hydrogenation of simple olefins. However, the reference clearly points out that not all olefin compounds could be successfully hydrogenated through the use of such homogeneous organolanthanide catalysts. In particular, tetramethylethylene could not be hydrogenated at a measurable rate by any of the organolanthanide catalysts disclosed in the foregoing references.
For the teachings contained in the foregoing U.S. patents the same are hereby incorporated in their entirety by reference thereto.